Postsynaptic membrane fragments, containing up to 50% of their protein as acetylcholine receptor (AChR), can be prepared from the electric tissue of Torepedo californica. The specific binding and ion-permeability control properties characteristic of AChR function at intact cholinergic synapses are retained in the isolated membrane fragments. In this project, the electron paramagnetic resonance technique of spin labeling will be used to characterize dynamic protein and lipid interactions in functional AChR-rich membrane vesicles. Spin labeled lipids and proteins will be used to probe the effects of receptor activation, ionic environment, temperature, and vesicle modification on the following dynamic membrane properties: Rotational mobility of the AChR, membrane fluidity, lateral phase separations, immobilization of specific lipids by proteins, and transverse and lateral diffusion of lipids. The long-range goal of this project is to identify the membrane interactions that are important in the mechanism of AChR-mediated ion permeability control and in the maintenance of lateral mobility restrictions at the synapse.